Typically, an overcurrent sense circuit of this type is used to source a portion of the current to a bypass side when an overcurrent flows through the circuit, thereby protecting current control elements such as power transistors, or to stop power from a power supply to a load to protect a battery.
FIG. 1 shows a constant-current power supply incorporating a prior overcurrent sense circuit 10, where an op-amp OP1 controls an NPN power transistor 11 to maintain a constant current to the load. The overcurrent sense circuit 10 is arranged so that a current sense resistor 12 is inserted in a conduction path drawing a load current, and is connected between a base and an emitter of an NPN power transistor 13 provided in a bypass circuit. Note that reference numerals 14-17 and 18 in the figure denote resistors and a zener diode, respectively. According to such an overcurrent sense circuit, when an overcurrent flows in the current sense resistor 12 due to a state of the load, for example, the power transistor 13 is turned on, so that a portion of the current from the power supply end flows as a collector current via the power transistor 13, thereby protecting the power transistor 11.
FIG. 2 shows another example of the overcurrent sense circuit that can be substituted for the one shown in FIG. 1; in this example, both inputs of an op-amp OP2 that forms a voltage comparator are connected across both ends of a current sense resistor 12a, while a reference power supply 19 is inserted on the negative input end of the op-amp OP2, so that when an overcurrent flows in the current sense resistor 12a, the op-amp OP2 is activated to output a sense signal.
A disadvantage of the overcurrent sense circuit shown in FIG. 1 is that about 0.7 volts is needed to turn on the bipolar transistor 13, therefore significant power is consumed at the current sense resistor 12. Furthermore, because the voltage between the base and emitter of the bipolar transistor has a negative temperature characteristic, the current sense resistor 12 must have a similar negative temperature characteristic in order to accurately sense an overcurrent, so that special resistor materials are required for the current sense resistor.
Additionally, with the arrangement shown in FIG. 2, a voltage comparator OP2 and reference power supply 19 with a very small offset voltage are required to sense a current at a low voltage drop. For example, if the voltage across the current sense resistor 12a is set to detect an overcurrent at 100 mV and an offset voltage of 10 mV is used for the voltage comparator OP2, then the current that can be actually sensed contains 10% error relative to the set current, even in consideration of the offset voltage only; when the offset voltage for the reference power supply is included, the error is further increased, so that the overcurrent cannot be sensed with a high degree of accuracy. Additionally, because the temperature coefficients for the reference power supply 19 and resistor 12a must be matched, there is small latitude in choice of the resistor 12a.
Thus, it is an object of the present invention to provide an overcurrent sense circuit that consumes less power and can sense an overcurrent with a high degree of accuracy.